1. Field of the Invention
This invention relates generally to quadrature oscillators, and more particularly, to a technique for maintaining low phase noise while modulating the amplitude of a quadrature oscillator.
2. Description of the Related Art
Quadrature oscillators based on two cross-coupled gm/C integrator stages provide high-frequency operation with low phase noise and are suitable for bipolar monolithic implementation. An example of such a quadrature oscillator is disclosed in U.S. Pat. No. 5,489,878 which is herein incorporated by reference. To assure low phase noise, the amplitude of such a quadrature oscillator must be limited so as to prevent a nearly perfect sinusoidal oscillation from becoming a large amplitude relaxation oscillation with high phase noise.
Some techniques for controlling the amplitude of a quadrature oscillator are described in the above-referenced U.S. Patent, as well as U.S. Pat. No. 5,714,911 which is also herein incorporated by reference. These techniques generally involve clamping the oscillation signal to a reference voltage or utilizing an outer control loop which senses the oscillation amplitude and then drives a positive or negative impedance cell responsive to the sensed amplitude so as to maintain a stable amplitude of oscillation.